Added currency provider for nl_NL

[MarcelRobeer]

What does this changes

Added a currency provider for Dutch (nl_NL).

What was wrong

faker dit not include the correct price format (Euro sign before value) for the nl_NL locale.

How this fixes it

• Added an nl_NL folder in faker/providers/currency/
• Added an __init__.py file in the nl_NL folder, with the same format as faker/providers/date_time/fr_FR/__init__.py
• Changed the format to place the Euro sign before the value, and not add a breaking space between the Euro sign and value

[fcurella]

Thank you!